Turbocharged internal combustion engines often have a waste-gate that can be opened to allow a portion of exhaust gasses to bypass the turbocharger's turbine. One way to actuate the bypass valve (also known as the waste-gate) is by an actuator that is comprised of a flexible diaphragm and spring. The spring applies a force to the diaphragm and lever connected to the diaphragm to keep the bypass valve closed when atmospheric pressure is present on both sides of the diaphragm. The valve can be opened by applying a gas pressure to the diaphragm. When the gas pressure overcomes the closing spring force, the waste-gate opens and a portion of exhaust gases bypass the turbocharger. One example of controlling a waste-gate in this way is described in U.S. Pat. No. 5,729,980. This patent describes a spring return pneumatically actuated waste-gate that is controlled based on engine speed. In particular, this turbocharger waste-gate actuator has two ports wherein gas pressure can act on the waste-gate diaphragm from either direction. This arrangement, when coupled with a throttle position sensor, is claimed to allow the waste-gate to remain closed for a longer period of time during full-throttle acceleration.
The above-mentioned method can also have several disadvantages. For example, the method simply controls the boost pressure depending on engine speed. In this arrangement, it may be possible to cause the spring assisted waste-gate valve to flutter or oscillate causing exhaust flow to the turbocharger to increase and decrease in an undesirable manner. This is especially true when the gas pressure used to open the bypass valve approaches the spring force that is applied to the diaphragm when the waste-gate is closed. In addition, this approach does not recognize or compensate part-to-part variations that can occur during manufacturing (e.g., spring rate variations) and that may result in uneven flow in dual turbocharger applications.
The inventors herein have recognized the above-mentioned disadvantages and have developed a method that offers substantial improvements.